A passive radio frequency identification system (RFID) may generally operate based on theories of mutual coupling and back scattering.
The theory of mutual coupling among antennas has application to a RFID system operating in a high frequency (HF) range such as an access authentication system or an access card system in transportations and securities. According to the theory of mutual coupling, information can be communicated between a tag and a reader antenna based on a proximity effect of electromagnetism so that the communication between them may not seriously influenced from dielectric materials around them. However, an interrogation range from the reader antenna to the tag is very short, and recognition of the reader antenna is performed not faster than another RFID system operating in an ultra high frequency (UHF) range.
The RFID system operating in an UHF range is depending on radiation and scattering characteristics of electromagnetic waves according to the theory of back scattering. The RFID system operating based on the theory of mutual coupling among antennas has a longer interrogation range and a faster recognition than that based on the theory of mutual coupling. Also, the RFID system according to the theory of back scattering can dramatically reduce a size of antenna.
However, since the RFID system according to the theory of back scattering has a lower recognition rate than that based on the theory of mutual coupling, there is a limit to wherever the RFID system according to the theory of back scattering can be widely used.
Recently, to increase a recognition rate of the RFID system, most researchers make the best of both the RFID systems operating based on theories of mutual coupling and back scattering in order to develop a reader antenna used in a UHF range at a near-field region.
To generate a magnetic field at an aperture of a conventional near-field antenna in a UHF range, coupled lines has a shorter length than that corresponding to a half-wavelength are arranged in a loop pattern, or a current flows at two patches in the opposite direction so that a strong magnetic field can be made between them in a vertical direction.
Accordingly, a near-field tag is easily recognized because of a strong magnetic field in a vertical direction, but a tag may not be recognized in a remote-field region.